Regenerative braking control system for electric vehicle

ABSTRACT

A regenerative braking control system for an electric vehicle which can be driven by operating an electric motor with electric energy supplied from an electric energy supply source mounted on the vehicle. A control unit (7) is designed so that regenerative braking force of the electric motor (2) is controlled in accordance with an inclination detected by an inclination detecting unit (28). This has made it possible to provide the electric vehicle with improved drivability and running performance.

TECHNICAL FIELD

This invention relates to an electric vehicle which runs by drivingwheels with an electric motor, and especially to a regenerative brakingcontrol system for an electric vehicle, which controls regenerativebraking by an electric motor in accordance with road conditions anddriver characteristics.

BACKGROUND ART

Electric vehicles (electric cars) have been attracting increasinginterest in recent years from the viewpoint of prevention of airpollution and reduction of vehicle noise. With these electric cars,so-called regenerative braking can be easily performed. Thisregenerative braking can be effected by changing over the operation modeof an electric drive motor (hereinafter called the "motor") into anelectric power generation mode, whereby rotational energy of the drivingwheels is recovered as electric energy through the motor.

Such regenerative braking is generally controlled so that upondepression of a brake pedal or upon release of an accelerator pedal froma depressed position, braking force is produced in association with thedepression or release.

When an accelerator pedal is released from a depressed position while abrake pedal is in a non-depressed position, regenerative braking weakerthan that exerted when the brake pedal is in a depressed position isperformed (this regeneration will be called "moderate regeneration") sothat the regenerative brake becomes equivalent to an engine brake in thecase of an automotive vehicle driven by an internal combustion engine.If more regenerative braking force than needed is produced at this time,the vehicle speed may, however, drop beyond the driver's desire.

Further, when regenerative braking is performed while driving at a lowvehicle speed, such as driving in an urban district, electric powersupplied to a motor becomes greater, thereby failing to achieve energysaving.

A technique that makes it possible, by a manual operation, to varymoderate braking force, which is equivalent to an engine brake andapplied upon release of an accelerator pedal or a brake pedal in anelectric car, has therefore been proposed, for example, in JapanesePatent Application Laid-Open (Kokai) No. HEI 5-122805.

According to the above-mentioned conventional technique of JapanesePatent Application Laid-Open (Kokai) No. HEI 5-122805, however,regenerative braking force is adjusted by a manual operation. Whiledriving on a slope, moderate regeneration may have to be changedfrequently in accordance with variations in grade. This imposes an extraburden on the driver.

With the foregoing problem in view, the present invention has as anobject to provide of a regenerative braking control system for anelectric vehicle, which makes it possible to obtain appropriateregenerative braking force in accordance with conditions of a road, onwhich the vehicle is running, driver characteristics, and the likewithout needing frequent manual operations.

DISCLOSURE OF THE INVENTION

A regenerative braking control system according to the present inventionfor an electric vehicle therefore comprises an electric energy supplysource mounted on the vehicle, an electric motor electrically connectedto the electric energy supply source and having a power output shaftconnected to a driving wheel of the vehicle, a driving state detectionunit including an inclination detecting unit for detecting aninclination of the vehicle when running, and a control unit forcontrolling regenerative braking force of the electric motor on a basisof detection information from the inclination detecting unit of thedriving state detection unit.

Owing to the above constitution, regenerative braking forcecorresponding to an inclination can be automatically produced so thatappropriate regenerative braking force can be obtained without anyparticular operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram focusing on essential functions ofa regenerative braking control system according to a first embodiment ofthe present invention for an electric car;

FIG. 2 is a diagram showing characteristics of base gains for settingbase regenerative braking force by the regenerative braking controlsystem according to the first embodiment of the present invention forthe electric car;

FIG. 3 is a diagram illustrating characteristics of increase/decreasegains for the correction of base regenerative braking force by theregenerative braking control system according to the first embodiment ofthe present invention for the electric car;

FIG. 4 is a diagram illustrating characteristics of increase/decreasegains for the correction of base regenerative braking force by theregenerative braking control system according to the first embodiment ofthe present invention for the electric car;

FIG. 5 is a diagram illustrating an explanation of characteristics ofincrease/decrease gains for the correction of base regenerative brakingforce by the regenerative braking control system according to the firstembodiment of the present invention for the electric car;

FIG. 6 is a flow chart for describing an operation of the regenerativebraking control system according to the first embodiment of the presentinvention for the electric car;

FIG. 7 is a flow chart for describing an operation of a modification ofthe regenerative braking control system according to the firstembodiment of the present invention for the electric car;

FIG. 8 is a functional block diagram focusing on essential functions ofa regenerative braking control system according to a second embodimentof the present invention for an electric car;

FIG. 9 is a block diagram depicting an essential constitution for aninclination-dependent correction and aroad-conditions-and-driving-state-dependent correction by theregenerative braking control system according to the second embodimentof the present invention for the electric car;

FIG. 10 is a diagram illustrating details of increase/decrease gains forthe road conditions and driving state for aroad-conditions-and-driving-state-dependent correction by theregenerative braking control system according to the second embodimentof the present invention for the electric car;

FIG. 11 is a block diagram showing a road conditions determining unitfor a driving-state-dependent correction by the regenerative brakingcontrol system according to the second embodiment of the presentinvention for the electric car;

FIG. 12 is a block diagram showing a driving characteristicsdetermination unit for a driving-state-dependent correction by theregenerative braking control system according to the second embodimentof the present invention for the electric car;

FIG. 13 is a flow chart illustrating a controlling operation ofregeneration by the regenerative braking control system according to thesecond embodiment of the present invention for the electric car;

FIG. 14 is a diagram describing an operation of the regenerative brakingcontrol system according to the second embodiment of the presentinvention for the electric car;

FIG. 15 is a functional block diagram focusing on essential functions ofa regenerative braking control system according to a third embodiment ofthe present invention for an electric car;

FIG. 16 is a diagram illustrating characteristics of factors for settingregeneration gains of the regenerative braking control system accordingto the third embodiment of the present invention for the electric car;

FIG. 17 is a diagram illustrating characteristics of factors for settingregeneration gains of the regenerative braking control system accordingto the third embodiment of the present invention for the electric car;and

FIG. 18 is a flow chart illustrating an operation of the regenerativebraking control system according to the third embodiment of the presentinvention for the electric car.

BEST MODE FOR CARRYING OUT THE INVENTION

Based on the drawings, a description will hereinafter be made about theembodiments of the regenerative braking control system according to thepresent invention for the electric vehicle.

(a) Description of the first embodiment

First, the first embodiment of the present invention will be describedwith reference to the drawings.

In the regenerative braking control system according to the firstembodiment for the electric vehicle, numeral 1 designates a battery asan electric energy supply source as shown in FIG. 1. This battery 1 canbe repeatedly charged by an external battery charger which the vehicleis not equipped with. Designated at numeral 2 is an electric motor(electric drive motor) which is supplied with electric power from thebattery 1, and driving wheels 4 are drivenly connected to a power outputshaft of the motor 2 via a transmission 3. Arranged between the battery1 and the motor 2 is an electric power conversion circuit 5 so thatelectric power from the battery 1 is adjusted to a desired level throughthe electric power conversion circuit 5 and is then supplied to themotor 2.

Further, the electric power conversion circuit 5 is controlled through amotor controller 6. According to this motor controller 6, a power outputof the motor 2 is controlled corresponding to a stroke of anunillustrated accelerator pedal through the electric power conversioncircuit 5. The motor controller 6 is internally provided with aregeneration control unit (controller) 7.

At this regeneration control unit 7, the state of regenerative brakingis controlled. Regenerative braking itself means, as is well known,braking in which the operation mode of the electric drive motor 2 ischanged over into an electric power generation mode, kinetic energy ofthe driving wheels 4 is recovered and the battery 1 is then chargedusing the kinetic energy as electric energy.

To control such regenerative braking, the regeneration control unit 7 isprovided with a storage unit 8, a determination unit 9, a computing unit10, and a command unit 11.

Connected to the regeneration control unit 7 is a driving statedetection unit 20, which includes a brake pedal stroke detection unit 21(which may be a brake switch) as a brake operation detection unit, anaccelerator pedal stroke detection unit 22 (which may be an acceleratorswitch), a motor torque detection unit 23, a vehicle speed detectionunit (vehicle-speed sensor) 24, a steering angle detection unit(steering angle sensor) 25, a motor rpm detection unit (rpm detectionunit) 26, a shift lever position detecting unit (forward/reversedetection unit) 27, an inclination detecting unit 28, and a regenerativetorque adjusting switch 29, whereby information on driving and operationstates--such as brake operation information, accelerator operationinformation, motor torque information, vehicle speed information,steering angle information, motor rpm information, vehicleforward/reverse information, road grade information, and regenerativetorque adjustment information--is inputted. Incidentally, theregenerative torque adjusting switch 29 is a manually-operated memberwhich makes it possible to adjust the strength of regenerative brakingby the driver's manual operation.

Stored in the above-mentioned storage unit 8 are, for example, variousvehicle data--such as weights W,WO, a frontal projected area S of thevehicle, a rolling resistance coefficient μr, drag coefficient μc,transmission gear ratio nt and final drive gear ratio nf--and aninclination-regeneration increase/decrease table (or map), arpm-regeneration table (or map), a table (or map) concerning corneringresistances Rc, and the like. These tables (or maps) will be describedsubsequently herein.

The determination unit 9 performs a determination with respect toregenerative braking on a basis of various information from theoperation state detecting unit 20. From shift position information fromthe shift position detecting unit 27, for example, the determinationunit 9 determines whether the vehicle is in an advancing state or in areversing state. Further, the computing unit 10 performs computation tocontrol regenerative braking. This computing unit 10 is provided with aregenerative braking force calculation unit (or computing unit) 12.

This regenerative braking force calculation unit 12 is provided with abase gain computing unit 13 for computing a base gain with respect tothe base regenerative braking force on a basis of detection informationfrom the brake operation detecting unit 21 and the rpm detection unit 26and also with a correction computing unit 14 for correcting the baseregenerative braking force, which has been computed by the base gaincomputing unit 13, in accordance with an increase/decrease gaincorresponding to an inclination of the running vehicle. The regenerativebraking force calculation unit 12 multiplies the regenerative brakingforce (regenerative torque), which corresponds to the strength ofregenerative braking instructed through the regenerative torqueadjusting switch 29, by the regeneration gain calculated by the basegain computing unit 13 and the correction computing unit 14, wherebyregenerative braking force (regenerative torque) is obtained.

Namely, this regenerative braking force calculation unit 12 sets aregeneration gain equivalent to an engine brake as will be indicated bythe following equation:

    Engine-brake-equivalent regeneration gain=base gain+increase/decrease gain

Among these parameters, the base gain is set so that, when moderateregeneration is performed without operation of a brake, for example, thebase gain is proportional to the rpm (revolution speed) of the motor 2as is shown in FIG. 2. Specifically, an rpm-regeneration table or map ofsuch characteristics as shown in FIG. 2 is stored in the storage unit 8and based on this table or map, a base gain is calculated from an rpm ofthe motor 2.

Further, the increase/decrease gain is set corresponding to the grade(grade resistance) of a road as shown in FIG. 3 or FIG. 4. The settingcharacteristics shown in FIG. 3 can be applied to all vehicles equippedwith ABS (antilock brake system). Concerning vehicles equipped with noABS, these setting characteristics can be applied to those havingdriving wheels on a frontal side relative to an advancing direction. Inother words, these setting characteristics can be applied when a frontwheel drive vehicle is moving forward or a rear wheel drive vehicle ismoving backward.

On the other hand, the setting characteristics illustrated in FIG. 4 canbe applied to automotive vehicles equipped with no ABS and havingdriving wheels on a rear side relative to the advancing direction.Namely, the setting characteristics illustrated in FIG. 4 can be appliedboth when a front wheel drive vehicle is moving backward and when a rearwheel drive vehicle is moving forward. According to the settingcharacteristics shown in FIG. 3, the increase/decrease gain is set on adecrease side (namely, at a decrease gain) in the case of an ascent hill(i.e., the grade resistance is positive), and the magnitude of thedecrease gain increases as the degree of the upgrade becomes greater. Onthe other hand, the increase/decrease gain is set on an increase side(namely, at an increase gain) in the case of a descent hill (i.e., thegrade resistance is negative), and the magnitude of the increase gainincreases as the degree of the upgrade becomes greater.

However, a dead zone is provided in a range around a grade resistance of0 (namely, for a level road and roads having grades close to that of thelevel road) so that the stabilization of control is assured. Further, aminimum limit and a maximum limit are also imposed on theincrease/decrease gain. When the degree of the upgrade becomes greaterthan a predetermined value, the increase/decrease gain is set to theminimum limit, in other words, the magnitude of the decrease gain takesa maximum value (=-100%). When the degree of the downgrade becomesgreater than a predetermined value, on the other hand, theincrease/decrease gain is set to the maximum limit, in other words, themagnitude of the increase gain takes a maximum value (=100%).Practically feasible control can therefore be performed.

According to the setting characteristics illustrated in FIG. 4, theincrease/decrease gain is set on a decrease side (namely, at a decreasegain) in the case of an ascent hill (i.e., the grade resistance ispositive), and the magnitude of the decrease gain increases as thedegree of the upgrade becomes greater. In the case of a descent hill(i.e.., the grade resistance is negative), on the other hand, theincrease/decrease gain is held at 0% as in the case of a level road. Itis for the following reasons that in the case of a descent hill, theincrease/decrease gain is set at 0% to effect no correction as describedabove.

Regenerative braking force is applied to driving wheels. When a frontwheel drive vehicle is moving backward on a descent hill or a rear wheeldrive vehicle is moving forward on a descent hill as shown in FIG. 5,its driving wheels are located on an upper side of the hill so thatregenerative braking force is applied to the upper wheels on the hill.As the upper wheels on the hill bear less vehicle weight, an increase inregenerative braking force involves a potential danger that the wheelscould be locked. Accordingly, when driving wheels, to which regenerativebraking force is applied, are located on an upper side of a hill, theincrease/decrease gain is held at 0% to inhibit any increasingcorrection.

In an ABS-equipped vehicle, the ABS itself acts to prevent locking ofwheels. It is therefore unnecessary to set the increase/decrease gain at0 as described above. The increase/decrease gain is therefore set asshown in FIG. 3. Even for such increase/decrease gains,inclination-increase/decrease regeneration tables or maps of suchcharacteristics, as shown in FIG. 3 and FIG. 4, are stored in thestorage unit 8 and based on one of the tables or maps, anincrease/decrease gain is calculated from an inclination. It is alsopossible to set an increase/decrease gain for a grade resistance byusing segment interpolation on the basis of particular 4 or more points(break points) in FIG. 3 or FIG. 4.

At the regenerative braking force calculation unit 12, the base gain andthe increase/decrease gain, which have been set from the motor rpm andthe grade resistance as described above, are added to calculate aregeneration gain, and the regenerative braking force (regenerativetorque) preset based on information from the regenerative torqueadjusting switch 29 or the like is multiplied by the regeneration gain,thereby obtaining new regenerative braking force (regenerative torque).The regenerative braking force (regenerative torque), which has beencalculated at the regenerative braking force calculation unit 12 of thecomputing unit 8 as described above, is then fed to the command unit 11via a first-order lowpass filter 15 as a device for inhibiting anyabrupt change in a preset value of regenerative braking force. Owing tothe arrangement of the first-order lowpass filter 15, the regenerativebraking force (regenerative torque) is prevented from an abrupt changeso that the control of regeneration can be performed withoutincongruousness.

A description will now be made with respect to the detection of a grade.According to the inclination detecting unit 28 in this embodiment, agrade is estimated by conducting computation on the basis of anequilibrium of force components concerning running of a vehicle.Specifically, during running of the vehicle, an equation of forceequilibrium can be established as will described hereinafter.

    F=Ra+R                                                     (1.1)

where

F: tire driving force or tire braking force transmitted through a tire,

Ra: acceleration resistance, and

R: running resistance.

Of these parameters, the tire driving force or tie braking force F canbe calculated based on a motor torque (which can be computed from acurrent command value) as shown by the following equations:

    Tire driving force F=Tm×gear ratio×gear efficiency÷dynamic loaded tire radius

    Tire braking force F=Tm'×gear ratio×gear efficiency÷dynamic loaded tire radius+Br                                     (1.2)

where

Tm: power running torque of the motor (computed from a current commandvalue),

Tm': regenerative torque of the motor (computed from a current commandvalue), and

Br: mechanical braking torque.

On the other hand, the running resistance R is expressed by R (θ, V) asa function of a road surface inclination θ and a vehicle speed V, andthis running resistance R(θ,V) is expressed by the following equation.

    R(θ,V)=W(μr·cosθ·sinθ)·μc·S·V.sup.2 +Rc                          (1.3)

where

W: gross vehicle weight,

S: frontal projected area of the vehicle,

μr: rolling resistance coefficient,

μc: drag coefficient, and

Rc: cornering resistance.

Storage of values, which correspond to steering angles, in the form of atable on the basis of real-car data makes it possible to determine thecornering resistance Rc on the basis of a steering angle (steering wheelangle), which is detected by the steering angle sensor 25, withreference to the table.

Further, the acceleration resistance Ra can be calculated based on avehicle acceleration a as shown by the following equation:

    Ra=(W+ΔW)·a/g

    ΔW=WO{Ec+Fc(nt·nf).sup.2 }

    Ec=g·Iw/(r.sup.2 ·WO)

    Fc=g·Im/(r.sup.2 ·WO)                    (1.4)

where

WO: empty vehicle weight,

a: vehicle acceleration,

g: gravitational acceleration [=9.8 (m/s²)],

nt: transmission gear ratio,

nf: final drive gear ratio,

r: dynamic loaded tire radius,

Iw: moment of inertia of rotating tire part, and

Im: moment of inertia of rotating motor part.

The rotating tire part includes a tire, a brake drum, an axle shaft andthe like, while the rotating motor part includes a motor rotor, aflywheel, a clutch and the like.

In addition, the vehicle acceleration a can be determined by thefollowing equation:

    a=Δ[(revolution speed of motor+gear ratio)×2πtire radius]/Δt                                          (1.5)

Incidentally, the unit of the revolution speed of the motor is[revolutions/second], the unit of the tire radius is [meter], and theunit of At is [second].

Further, when a revolution speed of the motor is expressed in terms of[rpm] unit, namely, motor revolutions per minute, the following equationcan be obtained:

    Revolution speed of motor=motor revolutions÷60

Introducing the equation (1.3) into the formula (1.1), the followingequation can be derived:

    W·sinθ≈F-Ra-W·μc-μc·S.multidot.V.sup.2 -Rc                                             (1.6)

From these equations, a grade resistance W-sinθ or a grade θ can becalculated from motor torques Tm,Tm' determined from a current commandvalue to the motor 2, a vehicle acceleration a determined from adetection value or the like of the motor rpm sensor 26, and a corneringresistance Rc which can be determined based on a steering angle detectedby the steering angle sensor 25.

Now, a front wheel drive vehicle equipped with no ABS is taken as anexample. According to the regenerative braking control system of thisembodiment for the electric vehicle, the determination of regenerativebraking force or regenerative torque in regeneration control isperformed at predetermined intervals as shown in the flow chart of FIG.6.

Describing specifically, as is illustrated in FIG. 6, upon detection ofgrade resistance by the inclination detecting unit 28 in step S111, itis then determined in step S112 by the determination unit 9 on the basisof information from the shift lever position detecting unit 27 whether avehicle is moving backward. If the vehicle is found to be movingbackward by this determination, the regenerative braking forcecalculation unit 12 performs calculation of an increase/decrease gain instep S113 on the basis of the characteristics shown in FIG. 4.

When the calculation of the increase/decrease gain is performed based onthe characteristics of FIG. 4 as described above, an increase in thedegree of upgrade leads to a corresponding increase in the magnitude ofa decrease gain. In the case of an ascent hill, as the grade becomesgreater, the vehicle tends to be more decelerated due to gravity and theneed for regenerative braking force is reduced accordingly. When thedecrease in regeneration gain is with the increase in upgrade asdescribed above, the vehicle is allowed to run backward.

In the case of a descent hill, the increase/decrease gain is set at 0 sothat regenerative braking force is maintained at a level similar to thatapplied on a level road. Even when the driving wheels (regenerativelybraked wheels) are located on an upper side of the descent hill, bearless weight and have a potential danger of locking by an increase inbraking force, the above setting of the increase/decrease gain can avoidan increase in the regenerative braking force, that is, locking of thewheels, leading to an advantage that the running stability of thevehicle can be maintained.

As the increase/degrease gain is set at 0 in the gentle grade rangearound that of a level road, there is another advantage thatregenerative braking force can be stably obtained withoutincongruousness.

If the vehicle is not moving backward, on the other hand, calculation ofan increase/decrease gain is performed by the regenerative braking forcecalculation unit 12 in step S114 on the basis of the characteristicsshown in FIG. 3. When the increase/decrease gain is calculated based onthe characteristics of FIG. 3 in this manner, an increase in the degreeof upgrade leads to a corresponding increase in the magnitude of adecrease gain. In the case of an ascent hill, as the grade becomesgreater, the vehicle tends to be more decelerated due to gravity and theneed for regenerative braking force is reduced accordingly. When thedecrease in regeneration gain is increased with the increase in upgradeas described above, production of excessive regenerative braking forcecan be avoided so that the vehicle is allowed to smoothly run on theascent hill without incongruousness.

On the other hand, an increase in the degree of downgrade leads to acorresponding increase in the magnitude of an increase gain. In the caseof a descent hill, as the grade becomes greater, the vehicle tends to bemore accelerated due to gravity and the need for regenerative brakingforce becomes higher accordingly. When the increase in regeneration gainis increased with the increase in downgrade as described above,regenerative braking force is increased as needed so that the vehicle isallowed to smoothly run on the descent hill without incongruousness.

As the increase/degrease gain is of course set at 0 in the gentle graderange around that of a level road, there is a further advantage thatregenerative braking force can be stably obtained withoutincongruousness.

Subsequent to the determination of the increase/decrease gain asdescribed above, the regenerative braking force calculation unit 12, instep S115, adds the increase/decrease gain to a base gain and calculatesan engine-brake-equivalent regeneration gain (moderate regenerationgain).

An output from the regenerative braking force calculation unit 12 isprocessed through the first-order lowpass filter 15. Owing to thefirst-order lowpass filter 15, the regenerative braking force(regenerative torque) is prevented from abruptly changing so that instep S116 and onwards, the control of regeneration can be performedwithout incongruousness.

Described specifically, it is determined in step S116 whether or not themoderate regeneration gain is greater than 100%. If the moderateregeneration gain is greater than 100%, the routine advances to stepS117 and the moderate regeneration gain is set at 100% again.

Further, it is determined in step S118 whether or not the moderateregeneration gain is smaller than 0%. If the moderate regeneration gainis smaller than 0%, the routine advances to step S119 and the moderateregeneration gain is set at 0% again.

After subjection to such processing, the moderate regeneration gain isprocessed through the first-order lowpass filter 15 in step S120.

According to the system of this embodiment, the engine-brake-equivalentmoderate regenerative braking force becomes stronger automatically whenthe accelerator is released in the course of running on a descent hill.It is therefore possible to reduce the frequency of operations of thebrake pedal and that of operations of the regenerative torque adjustingswitch by the driver and owing to an improvement in the efficiency ofregeneration, also to increase the distance covetable per charging.

When the accelerator is released in the course of running on an ascenthill, the engine-brake-equivalent moderate regenerative braking forcebecomes weaker automatically. Accordingly, it is also possible to reducethe frequency of operations of the brake pedal and that of operations ofthe regenerative torque adjusting switch by the driver and owing to animprovement in the efficiency of regeneration, also to increase thedistance covetable per charging.

Further, both while moving backward on a descent hill in the case of afront wheel drive vehicle and while moving forward on a descent hill inthe case of a rear wheel drive vehicle, it is possible to prevent theregenerative braking force from increasing upon release of theaccelerator. Locking of the rear wheels relative to the runningdirection (i.e., the driving wheels in each case) can be prevented,leading to a still further advantage that the steering ability of thevehicle can be assured.

Incidentally, it may be contemplated to set, as shown by the chain linein FIG. 4, the increase/ lo decrease gain for a time that a front wheeldrive vehicle is moving backward on a descent hill or a rear wheel drivevehicle is moving forward on a descent hill. Namely, when theaccelerator is released while running on such a descent hill, themoderate regenerative braking force is increased less than those inother cases (see FIG. 3), thereby making it possible to avoid locking ofthe rear wheels (i.e., the driving wheels in each case) relative to therunning direction although the moderate regenerative braking force oftenrequired upon release of the accelerator while running on the descenthill is made somewhat stronger automatically. Of course, it is preferredfor such by setting first taking the prevention of locking of the wheelsinto consideration and then considering increasing the regenerativebraking force.

In such regenerative braking, as the grade becomes greater in the caseof an ascent hill, the vehicle tends to be more decelerated due togravity and the need for regenerative braking force is reducedaccordingly. As the upgrade becomes greater, the decrease in theregeneration gain is therefore increased to avoid production ofexcessive regenerative braking force. As the grade becomes greater inthe case of a descent hill, on the other hand, the vehicle tends to bemore accelerated due to gravity and the need for regenerative brakingforce is increased accordingly. As the upgrade becomes greater, theincrease in the regeneration gain is therefore increased so that theregenerative braking force can be increased as needed. By the way, thefriction resistance of tires is low on a road surface which is wet withrain or snow. Accordingly, an abrupt increase in regenerative brakingforce may result in locking of the tires or an abrupt depression of theaccelerator pedal may lead to slipping of the tires. It is thereforenecessary to consider increasing or decreasing the regenerative brakingforce on such a low μ-road.

Reference is hence made to FIG. 7, which is a flow chart illustratingoperation of regeneration control which takes such road conditions intoconsideration. A description will now be made about this flow chart. Instep S131, it is determined whether or not the accelerator is off. Ifthe accelerator is off, the routine advances to step S132. If theaccelerator is on, the routine advances to step S136. Various controlsare then performed. Namely, it is determined in step S132 whether or nota deceleration of the vehicle is greater than a preset decelerationlimit value, i.e., 0.8 G in this embodiment and this decelerated statehas continued for at least a predetermined limit period, i.e., for 0.05second in this embodiment. If so, it is determined that the vehicle ison a descent hill and the tires are in a locked state. In this case, theregenerative braking force of the motor 2 is set at 0 in step S133, andin step 134, control is performed using, as a grade resistance detectedby the inclination detecting unit 28, a grade resistance detected rightbefore the deceleration of the vehicle exceeded 0.8 G, that is, beforethe tires were brought into the locked state. In this manner, when thetires are brought into a locked state by regenerative braking force ofthe motor 2 on a low μ-road of a downgrade, the regenerative brakingforce is changed to 0 to release the tires from the locked state. Thedriver can therefore control the vehicle by depressing the foot brake.

It is then determined in step S135 that, if the vehicle speed is nothigher than a preset threshold value, i.e., 5 km/h in this embodimentand this state has continued for a predetermined threshold period, i.e.,5 seconds in this embodiment, the locked state of the tires has beenreleased and the vehicle is in a substantially stopped state. Like theabove-described embodiment, control is thus performed using a detectionresistance from the inclination detecting unit 28.

On the other hand, if the conditions that the vehicle speed is nothigher than the threshold value (5 km/h) and this state has continuedfor the predetermined threshold period (5 seconds) are not found to bemet in step S135, the routine advances to step S140 to set theregenerative braking force of the motor 2 to 0, and in step S141, avalue of grade resistance detected right before the deceleration of thevehicle exceeded 0.8 G, that is, before the tires were brought into thelocked state is set as a grade resistance detected by the inclinationdetecting unit 28. The routine then returns.

If the accelerator is found to be on in step S131, on the other hand, itis determined in step S136 whether or not the acceleration of thevehicle is greater than a preset acceleration limit value, i.e., 0.4 Gin this embodiment and this accelerated state has continued for apredetermined limit period, i.e., 0.05 second. If so, it is determinedthat the vehicle is on an ascent hill and the tires are in a slippingstate. In this case, control is performed in step S137 by using, as agrade resistance detected by the inclination detecting unit 28, a graderesistance detected right before the acceleration of the vehicleexceeded 0.4 G, that is, before the tires were brought into the slippingstate. In this manner, when the tires are brought into a slipping stateby a depression of the accelerator pedal on a low μ-road of an upgrade,the control is performed by changing the regenerative braking force tothe strength before the slipping, whereby the vehicle can be controlled.

After that, if an acceleration is not found to be higher than a presetacceleration threshold value, i.e., 0.3 G in this embodiment and thisstate is found to have continued for at least a predetermined thresholdperiod, i.e., 1 second in this embodiment, the tires are determined in astate released from the slipping state, and the routine then returns.

If the tires are not found to have been released from the slipping statein step S138, the routine advances to step S139, where control isperformed using, as a grade resistance detected by the inclinationdetecting unit 28, a value of grade resistance detected right before theacceleration of the vehicle exceeded 0.4 G, that is, before the tireswere brought into the slipping state. The routine then returns.

In this embodiment, each deceleration or acceleration of the vehicle isdetermined from an rpm of the motor 2 detected by the motor rpmdetection unit 26. As an alternative, each driving wheel 4 may beprovided with a wheel speed sensor. In this embodiment, the control wasperformed by setting the regenerative braking force of the motor 2 to 0in step S133 so that the tires were released from the locked state. Instep S133, the regenerative braking force may, however, be set to reducethe regenerative braking force to such an extent that the driving wheelscan be released from a locked state.

(b) Description of the second embodiment

With reference to the drawings, a description will next be made withrespect to the second embodiment of the present invention.

Functions of essential elements in the system according to the secondembodiment will be described first. As is depicted in FIG. 8, theessential elements of the system are constructed similarly as in thefirst embodiment. In the second embodiment, the elements of similarconstructions as the corresponding elements in the first embodiment willbe identified by like symbols, and their detailed description is omittedherein.

In the second embodiment, the operation state detection unit 20described in the first embodiment is additionally provided with abraking frequency detection unit 30, and the regeneration control unit(controller) 7 also described in the first embodiment is furtherprovided with a road conditions determining unit 31 and a drivingcharacteristics determination unit 32. Further, the regenerative brakingforce calculation unit 12 arranged within the computing unit 10 isprovided with a road conditions-driving state factor setting unit 12Cand a regeneration command value calculation unit 12D in addition to abase gain setting unit 12A and an increase/decrease gain setting unit12B.

A description will first be made of the regenerative braking forcecalculation unit 12. The base gain setting unit 12A sets a base gain onthe basis of detection information from the brake pedal stroke detectionunit 21 and the motor rpm detection unit 26. The increase/decrease gainsetting unit 12B sets an increase/decrease gain corresponding to aninclination of the running vehicle. The road conditions-driving statefactor setting unit 12C sets a road conditions-driving state factor inaccordance with road conditions and driving characteristics. Theregeneration command value calculation unit 12D calculates aregeneration command value (a regeneration control amount for givingdesired regenerative braking force) on the basis of the base gain, theincrease/decrease gain and the road conditions-driving state factor setby the respective setting units 12A,12B,12C.

Further, the regenerative braking force calculation unit 12 sets anengine-brake-equivalent regeneration gain (hereinafter called a"moderate regeneration gain" or simply a "regeneration gain") asindicated by the following equation:

    Moderate regeneration gain=(base gain+increase/decrease gain)×road conditions-driving state factor                           (2.1)

Among these parameters, the base gain set by the base gain setting unit12A and the increase/decrease gain set by the increase/decrease gainsetting unit 12B are set in a similar manner as in the above-describedfirst embodiment: At the base gain setting unit 12A, a base gain iscalculated from a map such as that shown in FIG. 2 with respect to thefirst embodiment and at the increase/decrease gain setting unit 12B, anincrease/decrease gain is set corresponding to a grade of a road (graderesistance) by using the maps illustrated in FIG. 3 and FIG. 4 inconnection with the first embodiment.

A description will next be made with respect to the roadconditions-driving state factor set by the road conditions-driving statefactor setting unit 12C. As is shown in FIG. 8 and FIG. 9, at the roadconditions-driving state factor setting unit 12C, a roadconditions-driving state factor such as that shown in FIG. 10 is set inaccordance with road conditions and driving characteristics set based onvarious driving state data from the motor torque detection unit 23, thevehicle speed detection unit (vehicle speed sensor) 24, the steeringangle detection unit (steering angle sensor) 25, and the motor rpmdetection unit (rotation detecting means) 26, which are all arrangedwithin the driving state detection unit 20.

As road conditions, the road conditions determining unit 31 to bedescribed subsequently herein determines, based on a vehicle speed, amotor torque, a motor rpm, and a steering angle, the kind of a road onwhich the vehicle is running, that is, which one of an urban district, ahigh-speed road, a mountain road and a jammed road the vehicle isrunning in or on. As driving characteristics, on the other hand, thedriving characteristics determination unit 32 to be describedsubsequently herein determines whether the driving characteristics ofthe driver are relaxed or tense or in-between (normal).

Depending on the road conditions and driving characteristics determinedas described above, the road conditions-driving state factor is set asshown in FIG. 10. Specifically, when the driving characteristics arerelaxed, the road conditions-driving state factor is set slightlysmaller so that the degree of regeneration tends to be weakened. Whenthe driving characteristics are tense, on the other hand, the factor isset slightly greater so that the degree of regeneration tends to bestrengthened. When the driving characteristics are normal, the factor isset between the former factor and the latter factor. Further, on ahigh-speed road, the factor is set smaller so that the degree ofregeneration tends to be weakened. On a jammed road, the factor is notchanged so that the degree of regeneration remains in an intermediatestate. In an urban district, the factor is set slightly greater so thatthe degree of regeneration tends to be somewhat strengthened. On amountain road, the factor is set greater so that the degree ofregeneration tends to be strengthened.

Now describing the road conditions determining unit 31 and the drivingcharacteristics determination unit 31 in detail, the above-mentionedroad conditions determining unit (road conditions and traffic situationestimating unit) 31 performs determination of conditions of a road onwhich the vehicle is running (estimation of road conditions and trafficsituation) on the basis of a vehicle speed, motor torque or motor rpmand a steering angle as shown in FIG. 9. Specifically, thisdetermination of road conditions is performed as illustrated in FIG. 11.

Namely, from a vehicle speed VB and a steering angle δ, parametersindicating a running state of the vehicle, for example, a proportion ofrunning time, an average speed and an average lateral acceleration aredetermined. At this time, a motor torque and a motor rpm may be detectedtogether with the vehicle speed, and based on their detection values orbased on the vehicle speed, the motor torque and the motor rpm, theseparameters may be determined respectively.

Among these parameters, the average speed and average lateralacceleration are common values and are calculated by known methods. Onthe other hand, the term "proportion of running time" means theproportion [=Td/(Td+Ts)] of a running time Td in a total time Tall ofthe running time Td and the stopped time Ts (=Td+Ts). A proportion ofrunning time is calculated by counting a stopped time Ts if the vehiclespeed VB is not higher than a predetermined value (for example, 10 km/h)when an ignition switch is turned on or by counting a running time Td ifthe vehicle speed VB is higher than the predetermined value (forexample, 10 km/h) when the ignition switch is turned on.

Based on these proportion of running time, average speed and averagelateral acceleration, the degree of urban district characteristics, thedegree of high-speed road characteristics, the degree of mountain roadcharacteristics and the degree of jammed road characteristics isestimated. In this embodiment, a fuzzy inference is used for thisestimation. In the case of an urban district, for example, there arecharacteristics that the average speed is low and the proportion ofrunning time is intermediate. In the case of a high-speed road, thereare characteristics that the average speed is high, the proportion ofrunning time is large, and the integral of lateral acceleration is low.In the case of a mountain road, there are characteristics that theproportion of running time is small and the integral of lateralacceleration is large. In the case of a jammed road, there arecharacteristics that the average speed is low and the proportion ofrunning time is small. By setting a membership function and a fuzzy ruleon the basis of such characteristics, it is possible to estimate thedegree of urban district characteristics, the degree of high-speed roadcharacteristics, the degree of mountain road characteristics and thedegree Of jammed road characteristics, respectively.

As current road conditions, the road conditions determining unit 31determines the district or road, the degree of characteristics of whichis the highest among the degree of urban district characteristics, thedegree of high-speed road characteristics, the degree of mountain roadcharacteristics and the degree of jammed road characteristics.

At the above-mentioned driving characteristics determination unit 32,the driving characteristics of the driver are determined based on roadconditions determined by the road conditions determining unit 31, anaccelerator pedal stroke detected by the accelerator pedal strokedetection unit 22 and a braking frequency (i.e., the frequency ofbraking operations) detected by the braking frequency detection unit 30as illustrated in FIG. 8 and FIG. 9. At the braking frequencydetermination unit 30, a braking frequency can be determined bymultiplying the number of braking operations and/or the brake-operatedperiod, which has been found by the brake pedal stroke detection unit21, a brake switch or the like, with a running time.

Incidentally, the term "driving characteristics of the driver" as usedherein is defined as will be described next. Namely, a degree of relaxeddriving or a degree of tense driving--which indicates whether a driverprefers, for example, relaxed running featuring gentle accelerations anddecelerations and a relatively constant speed (such running will becalled "relaxed running") or tense running featuring quick accelerationsand decelerations and a relatively high speed (such running will becalled "tense running")--is used as the driving characteristics of thedriver. Such driving characteristics can be estimated based on physicalquantities which indicate a driving state of the vehicle.

However, the driving characteristics of the driver vary depending on thetraffic situation of the road on which the vehicle is running. Takinginto consideration the road conditions determined by the road conditionsdetermining unit 31 as described above, the driving characteristicsdetermining unit 32 therefore determines the driving characteristics ofthe driver on the basis of the road conditions and the above-describedphysical quantities indicating the driving state of the vehicle [namely,an accelerator pedal stroke, a vehicle speed, a longitudinalacceleration available by calculation from the vehicle speed, and alateral acceleration available by calculation from the vehicle speed andthe steering angle (steering wheel angle)] as illustrated in FIG. 12.

Described specifically, with respect to each of the physical quantities(accelerator pedal stroke, vehicle speed, longitudinal acceleration, andlateral acceleration) indicating a driving state of the vehicle,frequency analysis is performed by a known statistical method tocalculate the average value and dispersion of each physical quantity.

Based on these average values and dispersions of the individual physicalquantities and the road conditions and traffic situation determined fromthe estimated degrees of urban district characteristics, jammed roadcharacteristics and mountain road characteristics, the drivingcharacteristics of the driver can be estimated from a correlationbetween average values and dispersions of the individual physicalquantities, said average values and dispersions being characterized forindividual road conditions, and driving characteristics of drivers.

In this embodiment, a neural network is used for the estimation of thedriving characteristics. Namely, the average value and dispersion ofeach physical quantity become higher with the tension of the driving ofthe driver, whereas the average value and dispersion of each physicalquantity become lower with the relaxation of the driving of the driver.Of course, depending on each road conditions, a different standard isused for the evaluation of driving characteristics. The neural networkis therefore formed so that the correlation between the average valuesand dispersions of the individual physical quantities, which correspondto the respective road conditions, and the driving characteristics ofdrivers is expressed as an associative model.

The average values and dispersions of the individual physical quantitiesas well as the road conditions and traffic situation, determined fromthe estimated degrees of urban district characteristics, jammed roadcharacteristics and mountain road characteristics, are inputted to theneural network to determine the driving characteristics (the degree ofrelaxation or the degree of tension) of the driver.

It is of course designed to perform this estimation of the drivingcharacteristics by always inputting latest data as the individualphysical quantities and road conditions so that, even when the drivingcharacteristics of the driver change, the thus-changed drivingcharacteristics can be estimated promptly. Therefore, the degree ofrelaxation or the degree of tension obtained by the drivingcharacteristics determination unit represents an adequate estimation ofthe characteristics of the driver during actual driving.

The driving characteristics determination means 32 determines thedriving characteristics of each driver by ranking them into three stagesof relaxed driving characteristics, normal driving characteristics, andtense driving characteristics.

After determinations are performed at the road conditions determiningunit 31 and the driving characteristics determination unit 32,respectively, as described above, the road conditions-driving statefactor setting unit 12C can set, based on the results of thedetermination, a road condition-driving state factor corresponding tothe road conditions and the driving characteristics from a table such asthat illustrated in FIG. 10.

As indicated by the equation (2.1), the regeneration command valuecalculation unit 12D of the regenerative braking force calculation unit12 calculates a regeneration gain by adding a base gain and anincrease/decrease gain, which have been set from a motor rpm and a graderesistance, together and then multiplies the regeneration gain by theroad conditions-driving state factor for the road conditions and drivingcharacteristics, whereby a final target regeneration gain, that is,target regenerative braking force (regeneration command value) can beobtained.

Incidentally, where the regenerative torque adjusting switch 29 isarranged, the target regeneration gain obtained in accordance with theabove-mentioned equation (2.1) is multiplied by a gain preset by theswitch, so that a final target regeneration gain (target regenerativebraking force or regeneration command value) is obtained.

The regenerative braking force (regenerative torque), which has beencalculated by the regenerative braking force calculation unit 12 of thecomputing unit lo as described above, is then fed to the command unit 11through the first-order lowpass filter 15. As a consequence, theregenerative braking force (regenerative torque) is prevented fromchanging abruptly owing to the provision of the first-order lowpassfilter 15, thereby making it possible to perform regeneration controlwithout incongruousness.

In this embodiment, a grade is estimated by computation on the basis ofbalancing of force components relating to running of the vehicle in asimilar manner as in the above-described first embodiment. A detaileddescription of this feature is omitted herein.

Since the regenerative braking control system according to the secondembodiment of this invention for the electric vehicle is constructed asdescribed above, control of regenerative braking force (regenerationcommand value) in regeneration control, for example, of a front wheeldrive vehicle equipped with no ABS is performed at predeterminedintervals as illustrated in the flow chart of FIG. 13.

Described specifically, it is determined in step S210 whether or not theaccelerator pedal is off. If the accelerator pedal is on, noregeneration control is performed. If the accelerator pedal is off,however, it is then determined in step S220 whether or not the brakepedal is off. If the brake pedal is on, a regeneration command value isset corresponding to the stroke of the brake pedal (brake pedal stroke)(step S2120). If the brake pedal is off, on the other hand, the routineadvances to step S230 onwards so that control of moderate regenerativebraking equivalent to an engine brake is performed.

Namely, a motor rpm is first detected in step S230 and the routine thenadvances to step S240, where a base gain is set by the base gain settingunit 12A on the basis of the motor rpm (see FIG. 2 of the firstembodiment). In step S250, a grade is then detected by the inclinationdetecting unit 28 as described above. The routine thereafter advances tostep S260, where based on the grade, an increase/decrease gain is set bythe increase/decrease gain setting unit 12B (see FIG. 3 and FIG. 4 ofthe first embodiment).

Road conditions are then determined by the road conditions determiningunit 31 (step S270) and driving characteristics are determined by thedriving characteristics determination unit 32 (step S280). The routinethen advances to step S290, where based on the results of thesedeterminations, a road conditions-driving state factor is set by theroad conditions-driving state factor setting unit 12C (see FIG. 10).

In step S2100, as indicated by the above-described equation (2.1), theregeneration command value calculation unit 12D calculates aregeneration gain by adding the base gain and the increase/decreasegain, which have been set from the motor rpm and the grade resistance,together and then multiplies the regeneration gain by the roadconditions-driving state factor for the road conditions and drivingcharacteristics, whereby a final target regeneration gain is obtainedand target regenerative braking force (regeneration command value) isset.

After that, a regeneration command is performed in step S2110 so thatactual regeneration control is performed.

Incidentally, where the regenerative torque adjusting switch 29 isarranged, the target regeneration gain obtained in accordance with theabove-mentioned equation (2.1) is multiplied by a gain preset by theswitch, so that a final target regeneration gain is obtained.

As has been described above, the regeneration command is performed inaccordance with the regeneration command value set in step S2100 orS2120.

As a result, as is illustrated in FIG. 14, the regenerative brakingforce can meet not only a requirement from the grade of the road butalso a requirement from the road conditions and the preference anddriving characteristics of the driver.

In an urban district, for example, the speed is low with largevariations in acceleration. For a driver who performs tense driving, theregenerative braking force is strengthened so that significantvariations in acceleration can be adequately dealt with.

On a high-speed road, the speed is high with small variations inacceleration. For a driver who performs relaxed driving, theregenerative braking force is weakened at a greater rate, and for adriver who performs normal driving, the regenerative braking force isweakened at a smaller rate. Smooth regenerative braking conforming withsuch small variations in acceleration can therefore be performed.

On a mountain road, the speed is somewhat high with extremely largevariations in acceleration. For a driver who performs tense driving, theregenerative braking force is strengthened at a greater rate and for adriver who performs normal driving, the regenerative braking force isstrengthened at a smaller rate. Irrespective of the characteristics of adriver, it is therefore possible to cope with large variations inacceleration.

On a jammed road, the average speed is low and the proportion of runningtime is small. Smooth regenerative braking is therefore performed understandard regenerative braking force.

Of course, the control of the regenerative braking force according tosuch road conditions and driving characteristics is performed whiletaking an inclination into consideration. It is therefore possible toachieve regenerative braking under appropriate regenerative brake forcethat reflects the inclination, the road conditions and drivingcharacteristics.

Because the calculation of the increase/decrease gain is conducted basedon such characteristics as illustrated in FIG. 4, the magnitude of thedecrease gain increases as the grade of the ascent hill becomes greater.On the ascent hill, as the grade increases, it becomes easier for thevehicle to decelerate for gravity so that the need for regenerativebraking force is reduced correspondingly. When the decrease in theregeneration gain is increased with the grade of the ascent hill asdescribed above, production of excessive regenerative braking force canbe avoided so that the vehicle is allowed to smoothly run on the ascenthill without incongruousness.

In the case of a downgrade, the increase/decrease gain is set to 0 sothat the regenerative braking force is maintained at a level similar tothat for a level road. Even when the driving wheels(regeneratively-braked wheels) are located on an upper side of a slopeand bear less vehicle weight and an increase in braking force has apotential danger of inducing locking, the above setting of theincrease/decrease gain can avoid an increase in regenerative brakingforce and hence induction of wheel locking, leading to an advantage thatthe running stability of the vehicle can be maintained.

In the gentle grade range around the grade of a level road, theincrease/decrease gain is set at 0. This brings about an advantage thatincongruousness-free regenerative braking force can be stably obtained.

When the calculation of the increase/decrease gain is conduced based onthe characteristics of FIG. 3, the magnitude of the decrease gainincreases correspondingly with the degree of the upgrade as describedabove. On an ascent hill, as the grade increases, it becomes easier forthe vehicle to decelerate for gravity so that the need for regenerativebraking force is reduced correspondingly. When the decrease in theregeneration gain is increased with the grade of the ascent hill asdescribed above, production of excessive regenerative braking force canbe avoided so that the vehicle is allowed to smoothly run on the ascenthill without incongruousness.

Further, as the degree of the downgrade becomes greater, the magnitudeof the increase gain increases correspondingly. As the grade becomesgreater in the case of a descent hill, the vehicle is more easilyaccelerated for gravity and the need for regenerative braking force isincreased accordingly. If the increase of the regenerative gain isincreased with the magnitude of the downgrade, the regenerative brakingforce is increased as needed so that the vehicle is allowed to smoothlyrun on the descent hill without incongruousness.

Of course, in the gentle grade range around the grade of a level road,the increase/decrease gain is set to 0. This also brings about anadvantage that incongruousness-free regenerative braking force can bestably obtained.

By controlling the regeneration gain as described above, it is possibleto reduce the frequency of operations of an accelerator and/or a brake,to improve the efficiency of regeneration, and also to increase thedistance covetable per charging of the electric car.

Further, the processing of an output from the regenerative braking forcecalculation unit 12 through the first-order lowpass filter 15 preventsan abrupt change of regenerative braking force (regenerative torque) sothat the control of regeneration can be performed withoutincongruousness.

(c) Description of the third embodiment

The third embodiment of the present invention will next be describedwith reference to the drawings.

This third embodiment is different in the manner of setting of aregeneration gain from the above-described first and second embodiments.Except for this differences, the third embodiment is constructed likethe first and second embodiments. In the third embodiment, elementsconstructed as in the first and second embodiments will be identified bylike symbols, and their detailed description is omitted herein.

In this third embodiment, the driving state detection unit 20 isprovided, as shown in FIG. 15, with a vehicle operation detecting unit100, the vehicle speed detection unit (vehicle speed sensor) 24, thesteering angle detection unit (steering angle sensor) 25 as a corneringstate detection unit, the inclination detecting unit 28 and the like.

Incidentally, the vehicle operation detection unit 100 is composed ofthe brake pedal stroke detection unit (brake operation detection unit)21 and the accelerator pedal stroke detection unit 22.

Further, the control unit 7 is provided with the regenerative brakingforce calculation unit 12. Arranged within this regenerative brakingforce calculation unit 12 is a gain setting unit 121. The gain settingunit 121 sets a regeneration gain K in moderate regenerative brakingequivalent to an engine brake on the basis of various information of avehicle speed, a grade and a steering angle detected from the vehiclespeed sensor 24, the steering angle sensor 25 and the inclinationdetecting unit 28. Incidentally, the detection of the grade is conductedby a similar method as that employed in the above-described first andsecond embodiments.

As will be indicated below by the equation (3.1), the regeneration gainK is calculated as a product of two factors, Ka (first factor) and Kaθ(second factor).

    K=Ka·Kaθ                                    (3.1)

A description will hereinafter be made with respect to the setting ofthe first factor Ka and the second factor Kaθ. In the storage unit 8,maps (factor-setting tables) such as those shown in FIG. 16 and FIG. 17are stored. If both the brake and the accelerator are not determined tohave been operated by the determination unit 9 on the basis of detectioninformation from the vehicle operation detecting unit 100, the gainsetting unit 121 reads the first factor Ka and the second factor Kaθ andsets the regeneration gain K.

Here, this first factor Ka is set corresponding to steering angleinformation from the steering angle sensor 25. As illustrated in FIG.16, the first factor Ka is basically set smaller as the steering anglebecomes greater.

This setting was adopted in view of the possibility that, if anexcessively large regeneration gain is given during steering, thepositional stability of the vehicle would be impaired.

On the other hand, the second factor Kaθ is set in accordance with athree-dimensional map such as that shown in FIG. 17 on the basis ofvehicle speed information and gradient information available from thevehicle speed sensor 24 and the inclination detecting unit 28,respectively.

In this three-dimensional map, as is illustrated in the drawing, thesecond factor Kaθ is set greater as the vehicle speed becomes higher andthe second factor Kaθ is also set greater as the grade becomes smaller(namely, the downgrade becomes greater).

Incidentally, it is necessary to make the regeneration-gain-settingfactor greater when the vehicle speed is high. An actually-producedregenerative torque is however not completely consistent with thetendency shown in FIG. 17. This is attributed to the characteristics ofthe motor 2 that no large torque is generally produced in a high rpmrange. When drive force of the motor 2 is transmitted to the drivingwheels 4 via the transmission 3, the rpm of the motor 2 varies dependingon the speed change ratio even at the same vehicle speed, resulting in avariation in the torque to be produced.

When the first factor Ka and the second factor Kaθ are set as describedabove, the product of these factors Ka, Kaθ is set as the regenerationgain K in accordance with the equation (3.1) at the gain setting unit121.

Incidentally, the regeneration gain K, set as described above, issubjected to the following correction in accordance with detectioninformation from the vehicle operation detection unit 100.

If it is determined, by the vehicle operation detecting unit 100, thatthe accelerator is operated during moderate regenerative braking (whilethe accelerator is off and the brake is also off), the generation gain Kis decreased by a predetermined amount to perform a correction so thatthe regenerative braking force of the motor 2 is decreased.

Namely, when the accelerator is additionally operated during moderateregenerative braking, regenerative braking force greater than thatrequired by the driver is produced. In such a case, the moderate brakingforce is decreased as described above.

If it is determined by the vehicle operation detecting unit 100 duringmoderate regenerative braking that the brake has been operated, acorrection is performed to increase the regeneration gain K by apredetermined amount so that the regenerative braking force of the motor2 is increased.

Namely, when the brake is additionally operated during moderateregenerative braking, regenerative braking force which is actuallyobtained is smaller than that required by the driver. In such a case,the moderate braking force is increased as described above.

Such a correction of the regeneration gain K is effected by correctingat least one of the first factor Ka and the second factor Kaθ. Inpractice, the correction of the gain K is performed by rewriting themaps shown in FIG. 16 and FIG. 17, in other words, by updating thememory. Described specifically, when it is desired, for example, todecrease the correction gain K, rewriting of the maps shown in FIG. 16and FIG. 17 is performed by multiplying the factors Ka,Kaθ, which havebeen obtained from the maps, by a predetermined value (for example, 0.98or so).

In some instances, however, it may not be possible to set, by such acorrection alone, moderate regenerative braking force which fullyreflects the driver's preference in driving and road conditions. Thepresent system is therefore designed to prohibit any correction of theregeneration gain K, the correction being performed by rewriting themaps, when one of the following operation situations is determined fromdetection information from the brake pedal stroke detection unit 21, theaccelerator pedal stroke detection unit 22 and the steering angle sensor25:

(1) Subsequent to the occurrence of moderate regenerative braking byrelease of the accelerator pedal and release of brake pedal, the brakepedal was operated and the vehicle speed has then dropped substantially.

(2) Subsequent to the occurrence of moderate regenerative braking byrelease of the accelerator pedal and release of brake pedal, the brakepedal was operated and the steering angle has then changedsubstantially.

(3) Subsequent to the occurrence of moderate regenerative braking byrelease of the accelerator pedal and release of brake pedal, anoperation of the accelerator pedal has continued for a predeterminedperiod or longer.

(4) Subsequent to the occurrence of moderate regenerative braking byrelease of the accelerator pedal and release of brake pedal, anoperation of the accelerator pedal and an operation of the brake pedalhave been repeated frequently at short intervals.

The operation situation (1) includes, for example, such a case that thedriver has operated the brake pedal because of a deceleration of avehicle running ahead of his vehicle, a deceleration during jamming, ora deceleration, stopping or the like for a traffic light. Since there isnot much necessity for performing a correction of the regeneration gainin such a case, correction of the regeneration gain is prohibited.

As the operation situation (2), it is possible to imagine an operationof the brake pedal for a deceleration before a curve. Because thedeceleration is by the driver's intention in this case, correction ofthe regeneration gain is prohibited.

In the case of the operation situation (3), on the other hand, thebraking force of moderate regenerative braking is not insufficient butthe driver desires a re-acceleration. In such a case, correction of theregeneration gain is also prohibited.

The operation situation (4) indicates such a case that runningand-stopping are repeated, for example, on a jammed road. Because it isalso unnecessary to correct the gain of the moderate regenerativebraking in such a case, correction of the regeneration gain isprohibited.

By subjecting the regeneration gain K to correction or, when such aspecific operation situation is determined, by prohibiting correction ofthe regeneration gain K, it is possible to set a regeneration gain Kcompatible with the driver's preference and road conditions. This makesit possible to set braking force which does not give incongruousness tothe driver.

Further, the regenerative braking force calculation unit 12 sends theregeneration gain K, which has been obtained as described above, to thecommand unit 11 via the lowpass filter 15. By the command unit 11, themotor 2 is controlled so that regenerative braking force is obtainedcorresponding to the regeneration gain K.

As the regenerative braking control system according to the thirdembodiment of this invention for the electric vehicle is constructed asdescribed above, control of moderate regenerative braking force(regeneration command value) is performed at predetermined intervals,for example, in accordance with a flow chart such as that shown in FIG.18.

Described specifically, it is first determined in step S301 whether ornot the conditions of brake-off and accelerator-off are met. If theseconditions are found to be met, the routine advances to step S302onwards to perform moderate regenerative braking.

In step S302, factors Ka,Kaθ for the determination of a regenerationgain K are read from maps such as those shown in FIG. 16 and FIG. 17. Instep S303, the product of these factors Ka,Kaθ is set at theregeneration gain K.

Next, it is determined in step S304 whether or not the brake pedal hasbeen operated. If an operation of the brake pedal is detected, theroutine advances to step S305. If no operation of the brake pedal isdetected, the routine advances to step S320.

When the routine has advanced to step S305, a vehicle speed, a roadsurface inclination and a steering angle are inputted, and a timer isstarted in step S306. In steps S307 to S309, a period until the brakepedal is released is measured. In step S310, a vehicle speed, a roadsurface inclination and a steering angle at the time of the release ofthe brake pedal are inputted again.

In step S311 onwards, the regeneration gain is subjected to a correctionif based on the results of step S310, the moderate regenerative brakingforce is determined to require a correction. This correction is howeverprohibited if it is unnecessary to correct the moderate regenerativebraking force.

Namely, if neither an operation of the brake pedal for the predeterminedperiod nor a substantial drop in vehicle speed is determined in stepS311 and if no significant change in steering angle is determined instep S313, the values of the above-described maps are rewritten tocorrect the regeneration gain K. Specifically, the maps are rewritten tomake the values of the factors Ka, Kaθ greater so that a correction isperformed to increase the regenerative braking force.

If any one of an operation of the brake pedal for the predeterminedperiod, a substantial drop in vehicle speed and a significant change insteering angle is detected in step S311 or step S312, such a correctionof the regeneration gain K is not performed, and the regenerativebraking force is set in accordance with the regeneration gain K set instep S303.

If no operation of the brake pedal is detected in step S304, on theother hand, the routine advances to step S320, where it is determinedwhether or not the accelerator pedal has been operated. If theaccelerator pedal is found to have been operated, a vehicle speed, aroad surface inclination and a steering angle are inputted in step S321and the timer is then started in step S322. Further, in step S323 toS325, a period until the accelerator pedal is released is measured. Instep S326, a vehicle speed, a road surface inclination and a steeringangle at the time of the release of the accelerator pedal are inputtedagain.

In step S327 onwards, the regeneration gain K is subjected to acorrection if, based on the results of step S325 and S326, the moderateregenerative braking force is determined to require a correction. Thiscorrection is however prohibited if it is unnecessary to correct themoderate regenerative braking force.

Namely, if neither an operation of the accelerator pedal for thepredetermined period nor a substantial increase in vehicle speed isdetermined in step S327, the values of the above-described maps forsetting the factors Ka,Kaθ are rewritten in step S328 to correct theregeneration gain K. Specifically, such a correction is performed byrewriting the values of the maps to make the values of the factors Ka,Kaθ smaller so that a correction is performed to decrease theregenerative braking force.

By setting the moderate regenerative braking in accordance with thethree running conditions of a vehicle speed, a steering angle and a roadsurface inclination as described above, it is possible to set moderateregenerative braking force compatible with the driver's preference androad conditions. During an application of moderate regenerative braking,incongruousness-free feeling can therefore be obtained.

Further, the above control can reduce the frequency of operations of thebrake pedal and also the frequency of operations of the acceleratorpedal. Owing to such reductions in the frequency of operations of thebrake pedal and the frequency of operations of the accelerator pedal,the efficiency of regeneration is improved so that the distancecovetable per charging of the electric car can be increased.

In the above-described embodiment, the first factor Ka was set from asteering angle and the second factor Kaθ was set based on a vehiclespeed and an inclination. As an alternative, it may also be possible toset a first factor Ka' from a vehicle speed and to set a second factorkaθ' on the basis of a steering angle and an inclination and then setthe product of these factors Ka', Ka' as a regeneration gain K. Namely,it may be possible to modify the two-dimensional map of FIG. 16 intosuch a map as permitting the setting of the first factor Ka' from avehicle speed and also to modify the three-dimensional map of FIG. 17into such a map as permitting the setting of the second factor kaθ' froma steering angle and an inclination.

CAPABILITY OF EXPLOITATION IN INDUSTRY

An application of the present invention to an electric vehicle, whichruns by driving wheels with an electric motor, makes it possible tocontrol moderate regenerative braking, which is equivalent to an enginebrake of an automotive vehicle equipped with an internal combustionengine, in accordance with the driver's preference and road conditions.As a result, feeling during moderate regenerative braking of theelectric vehicle is improved and in addition, the efficiency ofregeneration is also improved so that the distance covetable percharging is also increased. Accordingly, the present inventioncontributes to improvements in the drivability and running performanceof such an electric vehicle, and is considered to have extremely highutility.

What is claimed is:
 1. A regenerative braking control system for anelectric vehicle, comprising:an electric energy supply source mounted onsaid vehicle; an electric motor electrically connected to said electricenergy supply source and having a power output shaft connected to adriving wheel of said vehicle; a driving state detection unit includingan inclination detecting unit for detecting an inclination of saidvehicle in running; and a control unit for controlling regenerativebraking force of said electric motor on a basis of detection informationfrom said inclination detecting unit of said driving state detectionunit wherein said control unit is provided with a storage unit fordetermining the regenerative braking force of said electric motor atleast in accordance with said detection information from saidinclination detecting unit, and said control unit obtains, from saidstorage unit, a signal indicative of said regeneration braking forcethat corresponds to said detection information, and controls theregenerative braking based on said obtained signal.
 2. The system ofclaim 1, wherein said control unit controls said electric motor toincrease regenerative braking force when said vehicle is determined tobe running on a descent hill on the basis of detection information fromsaid inclination detecting unit, and controls said electric motor todecrease regenerative braking force when said vehicle is determined tobe running on an ascent hill on the basis of detection information fromsaid inclination detecting unit.
 3. The system of claim 1, wherein saidcontrol unit performs said control to produce regenerative braking forceof the same strength as that produced upon running on a level road whensaid vehicle is determined to be running on a road of a grade in apredetermined grade range close to that of said level road on the basisof detection information from said inclination detecting unit.
 4. Thesystem of claim 1, wherein said control unit performs said control sothat a decrease or increase in said regenerative braking force producedat said electric motor is limited when said inclination is detected tobe an upward inclination or downward inclination of a predeterminedvalue.
 5. The system of claim 1, wherein said driving state detectionunit is provided with a brake operation detecting unit for detecting anoperated state of a brake of said vehicle and also with a revolutionspeed detection unit for detecting a revolution speed of said electricmotor; andsaid control unit is provided with a regenerative brakingforce calculation unit, which includes a base computing unit forcomputing base regenerative braking force on a basis of respectivedetection information from said brake operation detecting unit and saidrevolution speed detection unit, and also with a correction computingunit for correcting said base regenerative braking force, which has beencomputed by said base computing unit, on a basis of said inclination ofsaid vehicle in running, whereby based on a preset value of regenerativebraking force calculated by said regenerative force calculation unit,regenerative braking force of said electric motor is controlled.
 6. Thesystem of claim 5, wherein, when said vehicle is determined to berunning on a road of a grade in a predetermined grade range close tothat of a level load on the basis of detection information from saidinclination detecting unit, said regenerative braking force calculationunit performs said control so that said base regenerative braking forceis used as said preset value of regenerative braking force.
 7. Thesystem of claim 5, wherein said control unit is provided with a devicefor preventing said preset value of regenerative braking force, whichhas been outputted to an output terminal of said regenerative brakingforce calculation unit, from varying abruptly.
 8. The system of claim 1,wherein said inclination detecting unit calculates said inclination on abasis of running resistance obtained by subtracting accelerationresistance from driving force or braking force produced at said drivingwheel.
 9. The system of claim 1, wherein said driving state detectionunit is provided with an acceleration or deceleration detecting unit fordetecting an acceleration or deceleration of said vehicle, and when astate in which said acceleration or deceleration of said vehicledetected by said acceleration or deceleration detecting unit exceeds apreset acceleration or deceleration limit value continues for at least apredetermined period, said control unit performs said control by using,as said detection information from said inclination detecting unit, aninclination of said vehicle immediately before said accelerationdeceleration of said vehicle has exceeded said acceleration decelerationlimit.
 10. The system of claim 9, wherein said driving state detectionunit is provided with a vehicle speed detection unit for detecting aspeed of said vehicle, andwhen a state in which a deceleration of saidvehicle detected by said acceleration/deceleration detection unitexceeds a preset deceleration limit value continues for at least apredetermined period, said control unit decreases regenerative brakingforce of said electric motor and then performs said control by using, asdetection information from said inclination detecting unit, aninclination of said vehicle immediately before said deceleration of saidvehicle has exceeded said deceleration limit value; and subsequently,when a state in which a vehicle speed detected by said vehicle speeddetection unit is not higher than a preset vehicle speed base valuecontinues for at least a predetermined base period, said control unitperforms said control by using, as a current inclination of saidvehicle, detection information from said inclination detecting unit. 11.The system of claim 9, wherein, when a state in which an acceleration ofsaid vehicle detected by said acceleration/deceleration detection unitexceeds a preset acceleration limit value continues for at least apredetermined period, said control unit performs said control by using,as detection information from said inclination detecting unit, aninclination of said vehicle immediately before said acceleration of saidvehicle has exceeded said acceleration limit value; and subsequently,when a state in which an acceleration of said vehicle is not higher thana preset acceleration base value continues for at least a predeterminedbase period, said control unit performs said control by using, as acurrent inclination of said vehicle, detection information from saidinclination detecting unit.
 12. The system of claim 1, wherein saidcontrol unit is provided with a road conditions determining unit fordetermining conditions of a road, on which said vehicle is running, on abasis of detection results from said driving state detection unit,andsaid control unit controls said regenerative braking force of saidelectric motor on a basis of detection results from said inclinationdetecting unit and determined information from a road conditionsdetermining unit.
 13. The system of claim 12, wherein said driving statedetection unit is provided with a vehicle speed detection unit fordetecting a vehicle speed of said vehicle and a steering angle detectionunit for detecting a steering angle of said vehicle, andsaid roadconditions determining unit determines a kind of said road, on whichsaid vehicle is running, on a basis of said vehicle speed detected bysaid vehicle speed detection unit and said steering angle detected bysaid steering angle detection unit.
 14. The system of claim 1, whereinsaid control unit is provided with driving characteristics determinationunit for determining driving characteristics of a driver of said vehicleon a basis of detection results of said driving state detection unit,andsaid control unit controls said regenerative braking force of saidelectric motor on a basis of detection results from said inclinationdetecting unit and determined information from said drivingcharacteristics determination unit.
 15. The system of claim 14, whereinsaid driving state detection unit is provided with an accelerator pedalstroke detection unit for detecting a stroke of an accelerator pedal ofsaid vehicle and also with a brake operation detecting unit fordetecting an operated state of a brake of said vehicle, andsaid drivingcharacteristics determining unit determines said driving characteristicson a basis of said accelerator pedal stroke detected by said acceleratorpedal stroke detection unit and said operated state of said brakedetected by said brake operation detecting unit.
 16. The system of claim1, wherein said control unit is provided with a road conditionsdetermining unit for determining conditions of a road, on which saidvehicle is running, on a basis of detection results of said drivingstate detection unit and also with a driving characteristicsdetermination unit for determining driving characteristics of a driverof said vehicle on a basis of said detection results of said drivingstate detection unit, andsaid control unit controls said regenerativebraking force of said electric motor on a basis of detection resultsfrom said inclination detection unit and respective determinedinformation of said road conditions determining unit and said drivingcharacteristics determination unit.
 17. The system of claim 1, whereinsaid control unit performs said control such that said regenerativebraking force of said electric motor becomes equal to said regenerativebraking force read from said storage unit.
 18. The system of claim 1,wherein said driving state detection unit is provided with vehicleoperation detecting unit for detecting an operated state of said vehicleby a driver of said vehicle, andsaid control unit is provided with astorage unit for determining the regenerative braking force of saidelectric motor in accordance with detection results at said inclinationdetecting unit, said control unit reads from said storage unit saidregenerative braking force corresponding to said detection results, andthen performs said control so that said regenerative braking force ofsaid electric motor becomes equal to said regenerative braking forceread from said storage unit, and said control unit corrects said controlof said regenerative braking force on a basis of detection results formsaid vehicle operation detecting unit.
 19. The system of claim 18,wherein said operated state detection unit is provided with at least oneof a vehicle speed detection unit for detecting a vehicle speed of saidvehicle and a turning state detection unit for detecting a turning stateof said vehicle, andsaid control unit is provided with a storage unitfor determining said regenerative braking force of said electric motorin accordance with detection results at at least one of said vehiclespeed detection unit and said turning state detection unit and also atsaid inclination detecting unit, and said control unit reads from saidstorage unit said regenerative braking force corresponding to saiddetection results, and then performs said control so that saidregenerative braking force of said electric motor becomes equal to saidregenerative braking force read from said storage unit.
 20. The systemof claim 18, wherein said vehicle operation detecting unit is providedwith an accelerator pedal stroke detection unit for detecting anoperation of an accelerator of said vehicle and also with a brakeoperation detecting unit for detecting an operation of a brake of saidvehicle, andsaid control unit corrects a base control value for saidregenerative braking force of said electric motor so that saidregenerative braking force of said electric motor is decreased upondetermination of an operation of said accelerator on a basis ofdetection results from said inclination detecting unit, said acceleratorpedal stroke detection unit and said brake operation detecting unitunder regenerative braking in which neither said accelerator nor saidbrake is operated and that said regenerative braking force of saidelectric motor is increased upon determination of an operation of saidbrake on a basis of detection results from said inclination detectingunit, said accelerator pedal stroke detection unit and said brakeoperation detecting unit under regenerative braking in which neithersaid accelerator nor said brake is operated.